Electrocardiograph leads-off indicator

ABSTRACT

A “leads-off indicator” for an ECG apparatus for indicating that one or more of a plurality of ECG electrodes is not properly affixed to a patient and that that obviates the need for a conventional high frequency drive signal, but instead, employs common mode input noise as a drive signal to a reference electrode such that if one of the electrodes defining an ECG vector is not properly affixed, an increase in the ambient noise on an ECG vector associated with the detached electrode occurs as a detectable event. A first algorithm is used to identify whether or not the reference electrode itself is properly affixed to the patient&#39;s right leg and, if so, the common mode signal presented to the remaining limb electrodes becomes unbalanced should one of the limb electrodes not be properly connected to the patient. An impedance balancing circuit is provided for developing signals allowing identification of a lose electrode when the ECG system does not utilize a right leg electrode as a reference.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates generally to medical diagnostic apparatus, andmore particularly to indicator circuitry included in anelectrocardiograph (ECG) system for indicating whether one or more skinelectrodes is properly connected to a patient.

II. Discussion of the Prior Art

Electrocardiography is the recording, usually from electrodes on thebody surface, of the electrical activity of the heart during the cardiaccycle. Before each part of the heart contracts, there is a change in themembrane potential of the cardiac muscle cells, thus depolarizationprecedes contraction while repolarization follows and precedesrelaxation. The potential differences can be recorded from electrodes onthe body surface and the appearance of the recorded ECG depends on thesequence of depolarization and repolarization of the cardiac muscle massand the position of the recording electrodes. A typical ECG utilizes 12leads, such that 12 samples may be recorded with standard connectionsbetween the patient and the ECG machine. Interpretation of an ECG canprovide a very detailed picture of heart function but, obviously to dothis, requires considerable skill and experience. To properly understandand interpret ECG recordings, one must be able to understand the originof the cardiac vectors, know the axes of the 12 ECG leads and appreciatethe convention of normal vectors of depolarization and repolarization.

At rest, during diastole, the resting membrane potential cannot bedetected without puncturing the cell with a microelectrode in that itdoes not cause any current to flow in the extracellular fluid. When thecardiac action potential propagates through the tissue, current flows inthe extracellular fluid and the intracellular fluid, driven by thedifference between membrane potentials in resting and depolarized zones.The potential difference recorded is a vector quantity in that it hasboth magnitude and direction and conventionally may be represented by anarrow pointed towards the resting membrane, i.e., in the direction ofspread of electrical activity. The length of the arrow, of course,indicates the magnitude of the potential. By convention, if the electricvector is oriented toward the positive recording electrode, it isrepresented by an upward deflection of the ECG.

A triangle (Einthoven's Triangle) with the heart at its center is formedby placing recording limb electrodes on both arms and the left leg. TheEinthoven's Triangle is generally represented as an equilateral in thatthe trunk of the patient is a uniform volume conductor and the heartacts a point source of electric vectors situated at its center. Vector Iis defined as the potential difference between the right arm (RA)electrode and the left-arm (LA) electrode. Vector II is the potentialdifference between the RA electrode and the left-leg (LL) electrode.Vector III is the potential difference between the LA and the LLelectrodes. According to Einthoven's Law, only two such leads areindependent in that the third lead can be simply calculated from theother two.

An ECG system also employs chest leads. More particularly, in clinicalroutine use, six chest leads are used to record cardiac events under asingle electrode with respect to an “indifferent” electrode. Thisreference point is formed by connecting the RA, LA and LL electrodestogether with resistors, with the reference potential beingappropriately the middle point of the Einthoven's Triangle, sometimesreferred to as the Wilson Potential. Three additional vectors referredto as “augmented limb lead vectors” are based upon the Wilson Potential.The three limb electrodes, the six chest electrodes and three augmentedlimb electrodes total the twelve leads.

Many ECG machines incorporate a “lead-off indictor” to help identify ahigh-impedance ECG electrode patch. By providing such an indicator, amedical professional is able to quickly locate the source of a noisysignal and take appropriate steps to secure the lead patch to the skinof the patient. This reduces the amount of set-up and trouble-shootingtime involved with an ECG measurement. Most conventional leads-offindicators use simple impedance measurements to determine whether anelectrode is attached to the patient. Typically, the ECG machine appliesa relatively high frequency (e.g., 30 KHz) drive signal to the patientthrough the electrode affixed to the patient's right-leg (RL) electrode.The ECG machine then measures this signal through the other inputelectrodes to determine whether the electrodes are properly attached bycomparing the amplitude of the transduced 30 KHz signal to apredetermined reference.

This conventional approach of applying a high frequency drive signal tothe RL electrode has drawbacks when several medical devices are used inconjunction with a given patient. Often several ECGs and monitors areconnected to the patient at once, potentially causing errors in theleads-off indication if several such machines utilize the standard 30KHz excitation signal. This problem can be significantly worse withcertain pacemaker patients. Pacemakers from several manufacturers alsoutilize an excitation frequency near 30 KHz for deriving a rate-adaptivecontrol signal based upon minute ventilation. The 30 KHz drive signal isapplied by the pacemaker pulse generator circuitry as a carrier signalthat is modulated by respiratory activity. The modulation signal isproportional to minute ventilation which is a parameter that variespredictably with the level of patient activity.

When ECG machines with prior-art style leads-off indicators are usedwith pacemaker patients having a minute ventilation-based rate adaptivepacemaker, the leads-off indicator can cause pacing at the upper-ratelimit. In October 1998, the FDA's Center for Devices and RadiologicalHealth issued an alert, warning physicians of this interaction.

In addition to the affects on the device, telemetry interference at 30KHz can be significant and may cause the leads-off indicator to provideerroneous results. Thus, a need exists for a leads-off indicator for usein ECG equipment that can operate without interference from or withother medical devices being used with a given patient.

The present invention provides a leads-off indicator that does notrequire a 30 KHz excitation signal, but instead, utilizes informationfrom the common-mode input noise to determine whether an electrode isconnected to the patient. Nearly all ECG equipment operates inelectrical environments with high levels of power line noise, 60 Hzbeing the dominant common-mode signal on the input electrodes forequipment used in the United States and 50 Hz in Europe. By comparingthe relative noise between vectors, the RL output, and the common-modeinput voltage, the noise level can be triangulated to reveal a highimpedance electrode. By using the common-mode input noise instead of a30 KHz drive signal, compatibility of the ECG leads-off indicator withminute ventilation-based rate adaptive pacemakers is provided, as is ahigh immunity to interference from telemetry or other monitors.

SUMMARY OF THE INVENTION

In accordance with the present invention, a leads-off indicator for anECG machine may comprise a plurality of leads, each having askin-contacting electrode at one end thereof adapted for attachment to apatient's body at predetermined locations to thereby define a pluralityof ECG sensing vectors therebetween. One of the plurality of electrodesis selectively connectable to the patient's right leg as a reference.Sense amplifiers are connected to receive ECG signals and common-modenoise picked up by the skin-contacting electrodes, other than the RLelectrode. Circuitry is provided for comparing a difference between anaverage of output signals from the sense amplifiers associated with theRA, LA, LL limb electrodes and signals derived from the RL electrodewith a predetermined reference voltage for producing an outputindicative of whether the RL electrode is properly connected to thepatient. When it is determined that the RL electrode is properlyconnected to the patient, the circuitry is operative to apply a negativefeedback signal as a drive to the RL electrode, where the feedbacksignal is proportional to the level of common-mode noise present on theelectrodes other than the RL electrode. When it is determined that theRL electrode is not properly affixed to the patient, the resulting noisesignal picked up by the RL electrode is used by the aforementionedcircuitry used to compare the difference between an average of outputsignals from the sense amplifiers. Once the state of the RL electrode isconfirmed, it is possible to identify which, if any, of the limb andchest electrodes are not properly secured to the patient.

DESCRIPTION OF THE DRAWINGS

The foregoing features, objects and advantages of the present inventionwill become apparent to those skilled in the art from the followingdetailed description of a preferred embodiment thereof when consideredin conjunction with the accompanying drawings in which:

FIG. 1 is a circuit diagram illustrating the input stage for an ECGleads-off indicator;

FIG. 2 depicts by means of a circuit diagram the decision circuitry usedto determine if the right-leg electrode is properly connected to thepatient;

FIG. 3 is a similar circuit diagram of the decision circuitry used todetermine if the right-leg electrode is not properly connected to thepatient;

FIG. 4 is a block diagram of a leads-off algorithm for ECG sensingleads;

FIG. 5 is a block diagram of the decision circuitry used to detect thestatus of a chest electrode;

FIG. 6 is a circuit diagram of the impedance balancing circuitry usedwith the ECG inputs when the ECG system does not utilize a right-legelectrode;

FIG. 7 is a logic diagram of identification circuitry used with theimpedance balancing circuitry of FIG. 6 to provide an indication of aparticular electrode that is not in proper contact with the skin of thepatient; and

FIG. 8 is a block diagram of an arrangement for use in ECG equipment foreliminating a contribution of a disconnected electrode to the right legfeedback path.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Typical ECG machines use the right-leg (RL) electrode to providenegative feedback to the patient to greatly improve common-moderejection. If the RL electrode is not properly attached to the patient,multiple channels can become noisy and corrupt the ECG traces. Leads-offindicators that rely upon excitation signals to determine lead statuscan develop an intermediate state if the RL electrode is not attached tothe patient. If the excitation signal does not reach the patient, itwill not be received by the input electrodes and will thereforeimproperly indicate that all of the input electrodes have a highimpedance condition. The present invention improves over such prior artdesigns by determining when only the RL electrode is removed from thepatient. This aids the user by providing a clear indication of theparticular electrode that needs adjustment.

As will be further explained, the leads-off indicator of the presentinvention utilizes two criteria to detect the status of the RLelectrode, depending on whether it is currently properly attached to thepatient or not. Indicated generally by numeral 10 in FIG. 1 is a circuitdiagram of the ECG's input stage. A plurality of electrodes 12, 14, 16and 18 are adapted to be attached to the patient, namely to the LL, LA,RA and RL, respectively. Each of the electrodes is connected by a leadto the non-inverting input of an operational amplifier. The LL electrodeis connected to the non-inverting input of amplifier 20, the LAelectrode to amplifier 22, the RA electrode to amplifier 24 and the RLelectrode to a single-pole, double-throw electronic switching device 26.Depending upon the state of the switch 26, the RL electrode functionseither as an input to operational amplifier 28 or as an output from aninverting amplifier 30.

Amplifiers 20, 22 and 24 have their output terminals coupled byresistors 32, 34 and 36 of equal value to a node 38, such that anaverage value of the outputs from the amplifiers 20, 22 and 24 isdeveloped at that node and is applied as an input to the invertingamplifier 30.

When RL electrode 18 is not attached to the patient, the switch 26disconnects the electrode from the output of inverting amplifier 30 and,instead, connects the RL electrode to the input of the sensing amplifier28. As can be further seen from FIG. 1, the amplified input signals fromelectrodes LL, RA, LA and RL are applied as inputs to the leads-offdecision circuitry illustrated in FIGS. 2 and 3.

Consider first the case where the ECG system is configured to operatewithout the RL electrode, but senses when the RL electrode is attachedto the patient. The switch 26 (FIG. 1) automatically disconnects the RLelectrode from the output of drive amplifier 30 and connects it to thesensing amplifier 28 when the RL electrode is not in use. The outputfrom amplifier 28 is applied to the inverting input of a differentialamplifier 40, while the sum of the outputs from amplifiers 20, 22 and 24is applied by the summing circuit 42 to the non-inverting input of thedifferential amplifier 40. It can be seen that under thesecircumstances, the RL input voltage is subtracted from the average ofthe LL, LA and RA signals. This average is commonly referred to as the“Wilson Potential” (V_(W)). When the RL electrode is not properlyconnected to the patient, 60 Hz noise on the RL lead and V_(W) will notbe correlated in phase and amplitude due to the differences inskin-electrode impedances and coupling strengths. The amplitudedifferences between these two signals are often an order of magnitude orgreater. However, once the RL electrode is properly attached to thepatient, the 60 Hz signals will be very similar in terms of amplitudeand phase, thus decreasing the difference between the inputs to thedifference amplifier 40 to a small value. The 60-cycle noise emanatingfrom a 60 Hz band pass filter 43 is converted to a DC signal by circuit44 and then low pass filtered at circuit 46 to isolate the DC offset.The resulting DC signal level is applied to the inverting input of acomparator 48 which has a predetermined voltage reference value from areference source 50 connected to its non-inverting input.

Once the difference between RL and V_(W) drops below the threshold levelestablished by reference 50, the output of the comparator is lowindicative that the RL electrode is, in fact, properly attached to thepatient. When this condition prevails, the system automatically throwsthe switch 26 to the output of the amplifier 30 and thereafter thedecision circuit of FIG. 3 is employed to detect when and if the RLelectrode becomes disconnected.

Referring, then, to FIG. 3, considering the RL electrode as beingconnected to the patient and functioning as an output driver, the levelof average 60 Hz noise is extremely small, typically below 10 μv, in anyof the input leads LA, RA or LL. Removing the RL electrode connection tothe patient results in a drastic increase in the value of the outputfrom the RMS-to-DC converter 44 causing the DC offset voltage at theoutput of the low pass filter 46 to exceed the reference voltage appliedto the comparator 48, resulting in its output going high, indicatingthat the RL electrode is not connected to the patient. At this point,the RL electrode connection is configured by the switch 26 as a sensinginput.

With the assumption that the RL electrode is properly connected to thepatient, the circuit arrangement of FIG. 4 can be used to identify aparticular one of the three limb electrodes, RA, LL or LA that is notproperly connected to the patient. As already mentioned, with the RLelectrode properly attached and thereby providing a negative feedbackdrive signal, the input 60 Hz noise is typically less than 10 μv inamplitude. If one of the sensing electrodes is removed from the patient,and thus removed from the RL negative feedback, the noise transduced onthe disconnected electrode will be significantly greater than the noisepresent on the other electrodes that are properly affixed. This causesgreater 60 Hz noise on the vectors associated with the detachedelectrode, leaving one valid ECG vector. For example, first consider thecase where the LA electrode comes loose from the patient while the LL,RA and RL electrodes maintain a good connection. Vector II (LL-RA)remains substantially free of noise since both electrodes definingVector II are attached to the patient. Correspondingly, Vectors I(LA-RA) and III (LL-LA) become noisy.

The circuit of FIG. 4 shows on implementation for identifying theparticular electrode that is not properly attached. The potentialdifference between the LA electrode and RA electrode comprises Vector I.The ambient noise is band pass filtered at 51 and the resulting noisesignal is converted to a DC level by an RMS-to-DC converter 52 with anyvestiges noise being removed by the low-pass filter 54, before beingapplied to the non-inverting input of a comparator circuit 56. Likewise,the potential difference between electrodes LL and RA define Vector IIand the noise signal present is passed by band pass filter 57 and isconverted to a DC level by RMS-to-DC converter 58. The output fromconverter 58 is low-pass filtered by circuit 60 and the resultingfiltered output signal is applied to the non-inverting input of acomparator 62. The potential difference between electrodes LL and LAdefine Vector III and it, too, is band pass filtered by circuit 63 andconverted to a DC level by RMS-to-DC converter 64. Its output signal islow-pass filtered by circuit 66 with the DC signal output from thefilter 66 being applied to the non-inverting input of comparator 68.Each of the comparators 56, 62 and 68 have the same reference orthreshold value applied over conductor 70 to the inverting inputsthereof If the outputs from comparators 56 and 62 are each high, ANDgate 72 will be enabled and its output signal will be indicative thatthe electrode RA is not properly affixed to the patient. If the outputsfrom comparators 62 and 68 are simultaneously high, gate 74 is enabledwhich is indicative that electrode LL is not positively affixed to thepatient. When the outputs from comparators 56 and 68 are simultaneouslyhigh, AND gate 76 will output a signal indicating that electrode LA isnot properly attached to the patient.

While FIG. 4 illustrates a simple implementation of the detectionalgorithm in an analog domain, those skilled in the art will recognizethat this algorithm may also be implemented in the digital domain. Thealgorithm can further be improved by incorporating a variable thresholdfor the comparators 56, 62 and 68 or by utilizing a digital signalprocessor to compare the magnitude values of each of the vectors.

Experiments have shown that if the disconnected electrode connects intothe feedback network to the RL electrode (see FIG. 1), any phasedifference between the noise picked up on the disconnected electrode andthe noise transduced from the patient may cause an error in the feedbackto the RL electrode, creating interference on a valid vector. However,the noise on the valid vector has been found to always remain much lessthan that on the other two vectors. Thus, by looking at the rawmagnitude values, triangulation using the two highest magnitudes above acertain threshold level improves the detection criteria.

Once it is determined that the RA, LA and LL electrodes are properlyaffixed to the skin of the patient, the leads-off indicator of the ECGsystem can also indicate whether a chest electrode (V) is properlysecured to the patient. ECG systems create a chest vector (V) bymeasuring the difference between the chest electrode V and the averagesignal at electrodes RA, LA and LL. Once the leads-off indicatorconfirms that all three limb electrodes are connected, the chestelectrode can be detected by comparing the 60 Hz noise on eachindividual vector to a fixed threshold. FIG. 5 illustrates a blockdiagram of a circuit capable of making the determination for one chestelectrode. This circuitry can be duplicated for as many chest vectors asare available in the system.

Referring to FIG. 5, the potential difference defining the chest vectorV is band pass-filtered by filter circuit 78 that has a pass-bandcentered on 60 Hz. It should be understood, however, that if the ambientnoise present in the environment is of a different frequency, e.g., 50Hz as it is in Europe, then the filter 78 would be designed to pass thatparticular frequency and attenuate frequencies above and below thecenter value. The output from the band pass filter 78 is converted to aDC signal by RMS-to-DC converter 80 and, again, the output of thatcircuit is low pass filtered by circuit 82, allowing the DC signalproportional to noise level to be applied to the non-inverting input ofoperational amplifier 84 configured as a comparator. If the output fromthe comparator 84 exceeds the threshold potential applied to thenon-inverting input of the comparator, the output from the comparatorwill be high indicative that the particular chest electrode is notproperly affixed to the patient's chest.

It is desirable in equipment, such as pacemaker programmers, to providean operational mode where the ECG feature thereof can function in theabsence of a RL electrode being attached to the patient while stillproducing clean, relatively noise-free ECG signals. As will be explainedin greater detail below, any skin-electrode impedance mismatch can becompensated for by incorporating an impedance balancing circuit into thesystem that automatically maximizes the common-mode rejection withoutrequiring an RL electrode connection. Because the present inventionprovides for automatically detecting whether the RL electrode isproperly attached to the patient, the ECG circuitry can automaticallyswitch between two modes of operation for optimal performance under bothconditions.

FIG. 6 illustrates an impedance balancing circuit designed to minimizecommon mode noise between two ECG electrodes by effectively adjustingthe resistive and reactive components of the input impedance associatedwith one of the two electrodes so as to effectively match the inputimpedance of the other. This circuit is similar in many respects to theautomatic input impedance balancing circuit described in currentlycopending application Ser. No. 09/561,063, filed Apr. 28, 2000, andentitled “Improved Automatic Input Impedance Balancing ForElectrocardiogram (ECG) Sensing Applications”, the contents of which arehereby incorporated by reference. A first ECG electrode connects througha current limiting resistor 100 and a phase lead network comprising theparallel combination of resistor 102 and capacitor 104 to thenon-inverting input of operational amplifier 106. Semiconductor diodes108 and 110 provide voltage surge protection to the ECG electrodes byclamping noise to a predetermined reference potential applied to thosediodes. The output V₁ appearing at node 112 is fed back through afeedback resistor 114 and an input resistor 116 to the non-invertinginput of the operational amplifier 106. An input capacitance 117 iscoupled between the non-inverting input of amplifier 106 and a source ofreference potential (ground).

In a similar fashion, a second ECG electrode is coupled through acurrent limiting resistor 118 and a series phase lead network comprisingresistor 120 in parallel with capacitor 122 to the non-inverting inputof a second operational amplifier 124. Again, diodes 126 and 128 areincluded for voltage surge protection of the downstream electronics.

In the case of the operational amplifier 124, its feedback circuitincludes a variable gain operational amplifier 126 whose output iscoupled, via input resistor 128, to the non-inverting input of amplifier124. The inverting input of voltage-controlled amplifier 126 isconnected to ground. A second voltage controlled amplifier 130 is alsoconnected in the feedback circuit of operational amplifier 124 and itsoutput is coupled through input capacitance 132 to the non-invertinginput of operational amplifier 124.

A differential amplifier 134 has its non-inverting input connected tothe node 112 at the output of operational amplifier 106. The invertinginput of differential amplifier 134 is tied to the output of amplifier124, via conductor 136. A voltage divider, including series connectedresistors 138 and 140, is coupled between the output terminals of theamplifiers 106 and 124. The common terminal 142 between the voltagedivider resistors 138 and 140 is directly connected to the invertinginput of a buffer amplifier 144 whose non-inverting input is tied toground. A feedback resistor 146 connects the output terminal ofamplifier 144 to its inverting input.

The outputs from the differential amplifier 134 and the buffer amplifier144 are connected through high-pass filters 148 and 150, respectively,with the resulting filtered output signals being applied to a multipliercircuit 152.

The output from the high-pass filter 148 is applied, via conductor 154,to a second multiplier circuit 156. Multiplier circuit 156 receives itssecond input through a 90° phase shift circuit that includes a phaseshift capacitor 158, a feedback amplifier 160, oppositely polledclamping diodes 162 and 164. The output from the amplifier 160 ishigh-pass filtered by filter circuit 166 and then applied to a secondinput of the multiplier circuit 156. The circuit 156 multiplies a 90°phase shifted version of the common mode signal developed at the outputof buffer amplifier 144 by the high-pass filtered ΔV signal at node 149.

The output from multiplier 152 is low-pass filtered by circuit 168 whoseoutput is then applied to an integrator circuit 170. The resultingoutput signal V_(r) is fed back over conductor 172 to the control inputof the voltage controlled amplifier 126. Likewise, the phase shiftedversion of the common mode signal outputted by multiplier 156 islow-pass filtered by circuit 174 and the resulting DC offset signal isintegrated by circuit 176 whose output passes over conductor 178 to thecontrol input of the voltage-controlled amplifier 130.

The impedance balancing circuitry illustrated in FIG. 6 is configured tomatch any series impedance mismatches that may be present betweenelectrode 1 and electrode 2. The circuit attempts to adjust theeffective resistance and reactance of impedance elements 128 and 132 sothat the attenuation and phase shift at the non-inverting input ofamplifiers 106 and 124 will be equal for common mode noise. As anexample, let it be assumed that the series impedance on electrode 1 isgreater than that of electrode 2. This causes a greater attenuation ofthe voltage applied to the non-inverting input of amplifier 106 than atthe corresponding input of amplifier 124. Hence, when the outputs fromthese two amplifiers (V₁ and V₂) are measured differentially bydifferential amplifier 134, a negative going signal ΔV will be outputtedby the differential amplifier 134. Likewise, if it is assumed that thereis a common mode 60 Hz noise signal on electrodes 1 and 2, thedifferential amplifier 134 will be a 60 Hz signal that is 180° out ofphase with respect to the common mode signal.

The feedback circuitry coupling the output from the differentialamplifier 134 to the voltage controlled amplifier 126 takes an averageof the input signals V₁ and V₂ which is the common mode signal that isdeveloped across the voltage divider 138-140 and which is buffered byamplifier 144 to become the signal V_(cm).

The common mode signal, V_(cm), is high-pass filtered at 150 to removeany DC offset, and is multiplied by the differential signal ΔV. In thecase of 60 Hz noise, in that it is 180° out of phase, the output fromthe multiplier 152 will be of a 120 Hz frequency with a negative DCoffset.

The DC offset is low-pass filtered at 168 to remove the AC signalcomponent leaving only the DC offset which is integrated by circuit 170to yield the control signal V_(r). With a negative DC input signal tothe integrator, V_(r) will begin moving toward the negative rail with aslope depending on the magnitude of the DC voltage input. This signaldrives the voltage-controlled amplifier 126 which is essentiallyconfigured in a boot strap relation with the input resistor 128 so thatthe voltage controlled amplifier 126 is varying the attenuation ofamplifier 124. Recalling that the original assumption has been that theskin electrode impedance or series impedance is greater on electrode 1than on electrode 2, there would be greater attenuation on the outputfrom amplifier 106 than that on amplifier 124. So that the common modeamplitudes at each input will be equal, the effective resistance ofresistor 128 must be decreased. The effect of the feedback signal V_(r)is to decrease the gain of amplifier 126, which functions to attenuateor decrease the effective resistance of input resistor 128 to cause itto become matched to the input resistance 116 of amplifier 106.

The feedback circuitry producing the control signal V_(c) at the outputof integrator 176 performs a similar function to the quadrature signalby altering the effective reactance of capacitor 132. Hence, for anycomplex impedance that is present at the inputs of amplifiers 106 and124, the feedback network described functions to balance the two.

Having described the impedance balancing network of FIG. 6,consideration will next be given as to how it may be used to provide aleads-off indication when the ECG system does not employ a RL electrode.If one of electrode 1 or electrode 2 is not properly connected to thepatient, there is an infinite impedance to the common mode signal andthe feedback circuit in the impedance balancing network will be unableto produce a balance. The feedback signals V_(r) and V_(c) will go allthe way to the rail voltage of the integrators 170 and 176 and willremain at that level. Further, it will be unable to null out the 60 Hznoise. If either the V_(r) and V_(c) goes to either its minimum ormaximum value, it is indicative that a lead is off

The impedance balancing circuit is duplicated. Electrode 1 in FIG. 6 maybe the RA electrode while electrode 2 may be the LL electrode. In theduplicated circuit, electrode 1 may be the RA electrode and electrode 2,the LA electrode. Under the assumptions made, the respective signalsV_(r) and V_(c) feedback signals are driving the LL and LA electrodes.Thus, if either signals V_(r) and V_(c) that is driving the LL electrodegoes to its rail, it would be known that vector II cannot be nulled out.Similarly, if signals V_(r) and V_(c) for the LA is also sitting at therail, then vector I cannot be nulled out. By triangulation, then, theparticular unattached electrode is identified.

FIG. 7 illustrates a block diagram of the decision circuitry used todetermine which electrode impedance is out of range and, therefore,which of the electrodes is not properly affixed to the patient. Thedecision circuitry includes a plurality of comparators 180-187, with theeven numbered comparators having their non-inverting inputs connected toa minimum reference voltage 188. The odd numbered comparators in FIG. 7have their inverting input terminals connected to a reference source 190which is the maximum rail voltage for the integrator circuits of FIG. 6.The signal developed at the output of the integrator 170 associated withthe LL electrode is connected to the inverting input of comparator 180and to the non-inverting input terminal of comparator 181. The signaloutput from integrator 176 in FIG. 6 for the LL electrode is connectedto the inverting input of comparator 182 and the non-inverting input ofcomparator 183. The output from integrator 170 associated with the LAelectrode is applied to the inverting input of comparator 184 and to thenon-inverting input of comparator 185. Finally, the output of integrator176 of FIG. 6 for the LA electrode is connected to the inverting inputof comparator 186 and to the non-inverting input of comparator 187. Theoutputs from comparators 180-183 are connected as inputs to an OR gate192 while the outputs from the comparators 184-187 are connected toinputs of an OR gate 194. Thus, if the signal signals V_(r) or V_(c)reaches the rail potential established by reference sources 188 and 190,one of the OR gates 192 or 194 will output an out-of-range signal. Atranslator shown enclosed by broken line box 196 is then used toidentify the particular electrode that is not properly connected to thepatient. If OR gates 192 and 194 are both outputting high signals, ANDgate 197 is enabled and it is the RA electrode that is improperlysecured to the patient. If only OR gate 192 is producing a high outputsignal, then AND gate 198 will be enabled to indicate that the LLelectrode is disconnected. If only OR gate 194 is producing anout-of-range signal, AND gate 200 will output a high signal indicatingthat the LA electrode is not properly attached.

While the cardiac rhythm management device programmer for which thepresent invention has been developed permits up to four sensingelectrodes (RA, RL, LL and V), clinicians may sometimes opt to use alimited subset of available electrodes. In a case where only two of thethree limb electrodes and the RL electrode are attached to the patient,the non-attached third limb electrode can couple noise into a displayedvector, via the RL feedback path. Besides alerting the clinician of adisconnected electrode, the ECG system can utilize the leads-offindicator to automatically eliminate the contribution of thedisconnected electrode to the RL feedback path, thereby maintainingoptimum performance regardless of the electrode configuration. To betterunderstand how this is achieved, reference is made to the block diagramof FIG. 8. Here, the input amplifiers 20, 22 and 24 associatedrespectively with the LL electrode 12, the LA electrode 14 and the RAelectrode 16 have their output terminals adapted for connection throughresistors 202, 204 and 206 of equal value by way of an electronicsingle-pole, single-throw switch 208 to the input of the feedbackamplifier 30. Consider first the case where a clinician only wishes todisplay vector II, i.e., the potential difference between the LL and RAelectrodes. If the ECG system is set up to allow viewing of multiplevectors, say, the three limb leads that provide vectors I, II and III,the switches 208 are closed such that the average of the LL, LA and RAinputs are fed back through amplifier 30 and the RL electrode to thepatient so as to provide attenuation of the common mode signal. In acase where the clinician only wants one ECG vector, he/she may onlyconnect up the LL and RA electrodes to the patient. This would leave theLA electrode unattached and it would not be at the same potential as theLL and RA electrodes. Hence, there will be a large voltage difference.This would be due to the fact that the LA electrode could be couplingnoise into the RL feedback signal thereby increasing the level of noiseon the ECG output. Since under the assumed conditions, LL and RAelectrodes now will not see the same level of noise, there is no longera common mode signal between them. In an automatic ECG configuration,the leads-off indicator of the present invention functions to determinethat a lead is not properly connected and removes the contribution ofthe non-connected electrode to the RL feedback. Where the LA electrodeis not being used, the connection between the unused electrode and theRL feedback is interrupted by one of the switches 208 so that the noiseperformance is maintained automatically.

This invention has been described herein in considerable detail in orderto comply with the patent statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to the equipment and operating procedures, can beaccomplished without departing from the scope of the invention itself.

What is claimed is:
 1. A leads-off indicator for an ECG apparatuscomprising: (a) a plurality of leads, each having a skin contactingelectrode at one end thereof adapted for attachment to a patient's bodyat a predetermined location, to define a plurality of sensing vectorstherebetween, one of said electrodes selectively connectable to thepatient's right leg; (b) a plurality of sense amplifiers individuallyconnected to receive ECG signals and common-mode noise picked up by theskin contacting electrodes, but not ECG signals and common-mode noisefrom said one of said electrodes; and (c) means for comparing adifference between an average of output signals from said plurality ofsense amplifiers and signals derived from said one of said electrodeswith a predetermined reference voltage for producing an outputindicative of whether the said one of said electrodes selectivelyconnectable to the patient's right leg is properly connected to thepatient.
 2. The leads-off indicator of claim 1 and further includingmeans for applying a negative feedback signal to the said one of saidelectrodes when the output of the comparing means indicates that thesaid one of said electrodes is properly affixed to the patient.
 3. Theleads-off indicator of claim 2 and further including switching means forselectively connecting said negative feedback signal as a drive signalto the said one of said electrodes when the output of the comparingmeans indicates that the said one of said electrodes is properly affixedto the patient and for connecting the said one of said electrodes to thecomparing means when the output of the comparing means indicates thatthe said one of said electrodes is not properly affixed to the patient.4. A leads-off indicator for an ECG apparatus comprising: (a) aplurality of leads, each having a skin contacting electrode at one endthereof adapted for attachment to a patient's body at a predeterminedlocation, to define a plurality of sensing vectors therebetween, one ofsaid electrodes selectively connectable to the patient's right leg; (b)a plurality of sense amplifiers individually connected to receive ECGsignals and common-mode noise picked up by the skin contactingelectrodes, but not ECG signals and common-mode noise from said one ofsaid electrodes; (c) a difference amplifier having positive and negativeinput terminals and an output terminal, the positive input terminalconnected to receive a signal proportional to the average of outputsignals from said plurality of sense amplifiers, the negative inputterminal connected to receive the signals derived from the said one ofsaid electrodes, the output terminal carrying a signal proportional tothe difference therebetween; and (d) a signal comparator for comparing asignal related to the signal carried by the output of the differenceamplifier to a predetermined reference.
 5. The leads-off indicator ofclaim 4 and further including signal processing circuitry disposedbetween the output terminal of the difference amplifier and the signalcomparator.
 6. The leads-off indicator of claim 5 wherein the signalprocessing circuitry produces a DC signal related to ambient noisepicked up by the skin contacting electrodes.
 7. The leads-off indicatorof claim 4 and further including means for comparing an ambient noiselevel of each of the plurality of sensing vectors to determine which, ifany, of the skin contacting electrodes, other than the said one of saidelectrodes, is not in contact with the patient.